GHSA-2h3h-q99f-3fhc

### Impact

Arbitrary File Creation, Arbitrary File Overwrite, Arbitrary Code Execution

`@npmcli/arborist`, the library that calculates dependency trees and manages the `node_modules` folder hierarchy for the npm command line interface, aims to guarantee that package dependency contracts will be met, and the extraction of package contents will always be performed into the expected folder.

This is, in part, accomplished by resolving dependency specifiers defined in `package.json` manifests for dependencies with a specific name, and nesting folders to resolve conflicting dependencies.

When multiple dependencies differ only in the case of their name, Arborist's internal data structure saw them as separate items that could coexist within the same level in the `node_modules` hierarchy. However, on case-insensitive file systems (such as macOS and Windows), this is not the case. Combined with a symlink dependency such as `file:/some/path`, this allowed an attacker to create a situation in which arbitrary contents could be written to any location on the filesystem.

For example, a package `pwn-a` could define a dependency in their `package.json` file such as `"foo": "file:/some/path"`. Another package, `pwn-b` could define a dependency such as `FOO: "file:foo.tgz"`. On case-insensitive file systems, if `pwn-a` was installed, and then `pwn-b` was installed afterwards, the contents of `foo.tgz` would be written to `/some/path`, and any existing contents of `/some/path` would be removed.

Anyone using npm v7.20.6 or earlier on a case-insensitive filesystem is potentially affected.

### Patches

2.8.2 (included in npm v7.20.7 and above)

### Fix and Caveats

There are two parts to the fix:

1. Immediately prior to extraction, if the target folder is not a directory, it is moved aside. (If the installation fails, filesystem entries moved aside in this manner are moved back as part of the rollback process.)
2. The `children` map that represents child nodes in the tree is replaced with a case-insensitive map object, such that `node.children.get('foo')` and `node.children.get('FOO')` will return the same object, enabling Arborist to detect and handle this class of tree collision.

This second item imposes a caveat on case _sensitive_ filesystems where two packages with names which differ only in case may already exist at the same level in the tree, causing unpredictable behavior in this rare edge case. Note that in such cases, the `package-lock.json` already creates a situation which is hazardous to use on case-sensitive filesystems, and will likely lead to other problems.

If affected by this caveat, please run `npm update` to rebuild your tree and generate a new `package-lock.json` file.